Staring with the central box, notice that 9 must be formed in different ways in both the vertical and horizontal directions. There are only three ways this is possible using the numbers 1 to 9 without repetition: 1,2,6; 1,3,5; 2,3,4. Since we know 2 and 3 are not in the same column or row, we know 1 must be in the same column and row as 2 or 3. Column 4 already has a 1, so the 1 can be placed in column 6, row 5. To complete the sums to 9, the 5 and 6 can be placed. By attempting to form the sums to 15 and 21 you will find there is only one way to place the remaining numbers in the central box. The next leap of insight is that for every horizontal sub-row, the 9 must be 1+2+6, the 15 3+4+8 and the 21 5+7+9. And for each vertical sub-column, 9 = 1+3+5, 15 = 2+4+9, 21 = 6+7+8. Any attempt to deviate from this will quickly lead to repetition of digits. It’s still not straightforward, but it is at least possible to logically deduce the remaining cells, broadly starting with the top middle box and moving anti-clockwise.